This invention relates to an alkylation process. More particularly, this invention relates to the alkylation of aromatic hydrocarbons with olefins in a continuous, pressurized, reactive distillation process employing a solid alkylation catalyst system.
Linear alkylated aromatic compounds have many uses of significant commercial value. For example, alkylated light aromatic compounds, such as benzene and cumene, have value as gasoline octane enhancers. Aromatic compounds alkylated with long chain (that is, having greater than about 10 carbon atoms) linear olefins are commonly sulfonated to produce surfactants suitable for use in detergent manufacture.
The chemical reactions involving alkylation of aromatics with olefins have been studied for a long time. For example, U.S. Pat. No. 2,860,173 discloses the use of a solid phosphoric acid as a catalyst for the alkylation of benzene with propylene to produce cumene. More recently, the use of Friedel Crafts catalysts, especially aluminum chloride and certain natural zeolites and synthetic commercial sieves, as alkylation catalysts, has been taught.
Commercially, alkylation of aromatics is frequently carried out in reactive distillation processes associated with the reformulation of gasoline. However, there continue to be problems associated with commercial alkylation processes. These include low yields of the desired alkylated products, a tendency to produce poly-substituted aromatics, and catalyst xe2x80x9ccokingxe2x80x9d, that is, the building up of carbonaceous deposits and heavy organics on the catalyst surface, with resultant decrease in catalyst effectiveness and a need to shut the process down to regenerate. Most of these problems are directly related to the exothermic nature of the reaction, which has a tendency to be difficult to control. As a result there has appeared to have been a limit to the amount of aromatic hydrocarbon that can be practically introduced into the system, even when co-fed with the olefin introduction. Most commercial alkylation using HF alkylation technology employ an aromatic hydrocarbon to olefin mole ratio in the range of 4/1-8/1. More recently, it has been proposed in fixed bed solid acid alkylation processes to use molar ratios up to 30/1. The ability to adjust this molar ratio over a wider range without increasing the flow of aromatic hydrocarbon into the process can provide significant advantage in enabling the selective production of mono-alkylated product as opposed to the di-alkylated product, which is known to cause more rapid deactivation of solid acid catalysts. Minimizing the amount of poly-alkylated product using much higher molar ratios of aromatic hydrocarbon to olefin in the reaction zone holds the potential of helping improve catalyst effective lifetime.
It is clear that a need exists for a method of alkylation of aromatics with olefins, particularly straight chain olefins, that has high olefin conversion rates, a high selectivity for mono-substituted products and prolonged catalyst effectiveness.
This invention provides a solution to one or more of the problems described above. More particularly, the invention provides a process and a system useful in the preparation of mono-alkylated aromatic compounds by the solid acid-catalyzed reaction of aromatic hydrocarbons compounds with olefins, particularly low molecular weight, straight chain olefins.
In one aspect, the invention is a system comprising a reactive distillation column including a reactive zone, a first rectification zone at the top of the distillation column and a second rectification zone below said reactive zone and further containing a solid acid alkylation catalyst supported in the reactive zone. Positioned below and in communication with the reactive zone through the second rectification zone is a reboiler and means for withdrawing alkylated aromatic compound from the reboiler. Suitably positioned injectors allow for the controlled introduction of aromatic hydrocarbon and olefin feed streams into the reactive zone such that the reactants flow counter-currently to each other in the liquid phase.
In a second aspect, the invention is a continuous reactive distillation process that comprises introducing into a reactive zone, at a point in the distillation column just above the catalyst zone and below a first rectification zone, at least a portion of the olefin containing feedstock and introducing an amount of aromatic hydrocarbons into the reactive zone, at a point below the catalyst zone but above a second rectification zone where it may be refluxed into the reactive zone such that the aromatic hydrocarbon flows upward and contacts the olefin as the olefin liquid phase descends and flows through the catalyst in the reactive zone, whereby the olefin and aromatic hydrocarbon react, in the liquid phase, to form an alkylated aromatic compound.